1. Field of the Invention
The present invention relates to mobile communication systems in general and, more particularly, to a scheme for assigning receiver fingers of a multiple finger receiver to different offsets corresponding to multipath components.
2. Description of the Related Art
A mobile communications channel can rarely by modeled as purely line-of-site. Therefore, one must consider the many independent paths that are the result of scattering and reflection of a signal between the many objects that lie between and around the mobile station and the base station. The scattering and reflection of the signal creates many different xe2x80x9ccopiesxe2x80x9d of the transmitted signal (xe2x80x9cmultipath signalsxe2x80x9d) arriving at the receiving station with various amounts of delay, phase shift and attenuation. As a result, the signal received at the mobile station from the base station (and at the base station from the mobile station) is made up of the sum of many signals, each traveling over a separate path. Since these path lengths are not equal, the information carried over the radio link will experience a spread in delay as it travels between the base station and the mobile station. The amount of time dispersion between the earliest received copy of the transmitted signal and the latest arriving copy having a signal strength above a certain level is often referred to as delay spread. Delay spread can cause intersymbol interference (ISI). In addition to delay spread, the same multipath environment causes severe local variations in the received signal strength as the multipath signals are added constructively and destructively at the receiving antenna. A multipath component is the combination of multipath signals arriving at the receiver at nearly the same delay. These variations in the amplitude of the multipath components is generally referred to as Rayleigh fading, which can cause large blocks of information to be lost.
Resistance to multipath fading is a reason for using spread spectrum systems for wireless communications. Spread spectrum signals are pseudorandom and have noise-like properties when compared with the digital information data. Certain spread spectrum systems, such as code-division multiple access (CDMA) systems, spread the baseband data by directly multiplying the baseband data pulses with a pseudo-noise (PN) code, which is a binary sequence that appears random but can be reproduced by the intended receiving station. The PN code has a much higher pulse rate than the data pulse rate, and a single pulse of the PN code is called a chip. Spread spectrum signals are demodulated in part at the receiving station through cross-correlation with a locally-generated version of the PN code. Cross-correlation with the correct PN code de-spreads the spread spectrum signal and restores the modulated message to the narrower band of the original data, while cross-correlating the signal from an undesired user with the PN code results in a small amount of noise. Because spread spectrum signals are spread over a large bandwidth, only a small portion of the spectrum experiences fading at any given time. The resistance of spread spectrum systems to multipath fading can also be explained from the fact that delayed versions of the transmitted signal should be generally uncorrelated with the original PN code and will simply appear as noise from another uncorrelated user.
Spread spectrum systems, such as CDMA systems, however, can advantageously use the delayed versions of the transmitted signal. Spread spectrum systems exploit the multipath environment by combining the information obtained from several resolvable multipath components. In CDMA systems, the effects of multipath are combated and advantageously exploited by using a multiple-branch (RAKE) receiver. FIG. 1 shows a RAKE receiver 10 with four xe2x80x9cfingersxe2x80x9d 12a-d. The RAKE receiver 10 can be implemented using a CDMA Cell Site Modem ASIC provided by Qualcomm of San Diego, Calif. as well as the control thereof. The RAKE receiver 10 attempts to collect the delayed or offset versions of the original signal by providing parallel demodulators or fingers 12a-d. Each demodulator 12a-d uses a different amount of delay or offset corresponding to a multipath component of the signal from a particular antenna 14. Initially, processing circuitry 18 assigns a delay or offset corresponding to a multipath component to each demodulator 12a-d. Afterward, tracking loops 20a-d make adjustments to the assigned delay or offset for the demodulators 12a-d. In a current CDMA RAKE receiver, finger tracking loops 20a-d perform xe2x85x9 PN chip adjustments to the assigned offsets or delays of the demodulators 12a-d. Searcher circuitry 19 performs a search to find the strongest multipath components within a range of offsets or delays. The results from the searcher 19 are used for the initial finger assignments and/or for any finger re-assignments after a finger 12a-d is disabled. A combiner 22 combines the outputs from the demodulators 12a-d and outputs the combined signal to the remainder of the receiver 10. The receiver 10 includes other aspects which are not discussed. For example, the combined signal is subsequently decoded. Furthermore, the signal received at the antenna 14 which is demodulated as generally described above can undergo additional processing depending on the particular implementation. For example, base stations typically use non-coherent demodulation, and mobile stations typically use coherent demodulation.
Each demodulator 12a-d de-spreads the incoming signal using the PN code and the delay or offset assigned to the demodulator 1212a-d . As such, the demodulators 12a-d extract multipath components of the original signal. The use of the parallel demodulators 12a-d improves the signal-to-noise ratio (SNR) of the received signal for the given user and provides a statistical and power diversity gain because uncorrelated multipath components will fade independently. Ideally, multipath components are uncorrelated when the components are more than 1 PN chip (approximately 0.8138 microseconds in IS-95 CDMA) from each other. The finger tracking loop 20a-d for each demodulator 12a-d of the RAKE receiver 10 is designed to keep the assigned finger delay or offset synchronized with the delay or offset yielding the strongest finger energy for the multipath component being tracked. Typically, an early-late gate tracking mechanism adjusts the assigned delay or offset based on the difference in finger energy between an early hypothesis (less delay) and a late hypothesis (more delay). As such, each tracking loop 20a-d adjusts the delay or offset for its finger 12a-d toward the local maximum of the correlation between the PN code and the received spread signal. A multipath component of a finger 12a-d having a particular offset will be partially correlated with a multipath component of another finger 12a-d having a difference in offset of less than 1 PN chip. Due to the partial correlation between the multipath components, the fingers 12a-d could end up tracking the same multipath component. Because of the early-late gate tracking mechanism, the tracker 20 could potentially be affected by multipath components having a difference in offset of more than 1 PN chip. For example, if the tracker uses +/xe2x88x92xc2xc chip early/late correlation hypothesis, the tracker 20a-d could be potentially influenced by a multipath component that is 1xc2xc chips away. Thus, even demodulators 12a-d assigned to offsets or delays greater than 1 PN chip in difference can still end up tracking the same multipath component.
For ease of explanation, FIGS. 2a-c represent the finger strength depending on the PN chip offset (delay) for several simplified situations involving two multipath components A and B. FIG. 2a shows the output 28 representing the correlation between PN de-spreading codes and the received signal for two, unfaded multipath components A and B separated by a differential delay d. FIG. 2b represents multipath component B experiencing a fade while multipath component A is unfaded, and FIG. 2c shows multipath component A experiencing a fade while multipath component B is unfaded. As such, when the multipath components A and B are uncorrelated (i.e., separated by greater than 1 PN chip offset) and if one finger 12 is tracking a faded multipath component, another finger 12 is probably tracking an unfaded multipath component, thereby preventing data from being lost. Consequently, the parallel demodulators 12a and 12b increase the multipath diversity gain and the average SNR for the receiver 10.
Multipath components having delays that are within or about 1 PN chip of each other (approximately 0.8138 microseconds in IS-95 CDMA) are common and can cause duplicate finger assignments, degrading SNR and the multipath diversity gain for the RAKE receiver 10. In fact, as mentioned above multipath components having delays greater than 1 PN chip can even lead to duplicate finger assignments. The RAKE receiver 10 may experience difficulty in consistently resolving more than one unique multipath component per antenna at any given instant in a low delay spread environment because of the behavior of current finger tracking loops 20a-d . For ease of explanation, FIGS. 3a-c show simplified representations of fingers 12a and 12b assigned to two different resolved multipath components. The difference d between the PN offsets of the fingers 12a and 12b is relatively small, for example less than 1 PN chip (low delay spread environment).
In FIG. 3a, the multipath components tracked by the fingers 12a and 12b are unfaded, and the tracking loops 20a and 20b maintain the assignment of the fingers 12a and 12b. In FIG. 3b, however, the multipath component of finger 12b is fading, and the nearby component of finger 12a is unfaded. In response, the tracking loop 20b of finger 12b tends to move the assigned PN offset corresponding to a multipath component for the finger 12b towards the offset of the stronger multipath component assigned to the finger 12a. Similarly, in FIG. 3c, the finger 12a is tracking a multipath component that goes into a fade, and the nearby multipath component assigned to the finger 12b is not in a fade. The tracker 20a adjusts the PN offset assigned to the finger 12a toward the PN offset of the stronger component assigned to the finger 12b. Therefore, in a low delay spread environment, current systems can experience duplicate finger assignments (two fingers with the same assigned delay corresponding to the same multipath component).
Duplicate finger assignments are undesirable because they provide no multipath diversity gain as with independently fading multipath components and thereby no improvement in the average SNR. Current systems combat duplicate finger assignments by simply disabling one of the duplicate fingers as a finger 12a-d appears within a certain amount of delay or offset from the delay or offset of the other finger 12a-d . The system searches for another unassigned multipath component which may be the previously faded multipath component which has re-emerged and reassigns the duplicate finger to that multipath component. This scheme of disabling, searching and reassigning reduces the efficiency and effectiveness of the receiver, especially in the case of a continuously fading and re-emerging multipath component.
Accordingly, a need exists for a multiple finger receiver that effectively assigns the receiver fingers to multipath components in a manner which reduces the adverse effects of duplicate finger assignments.
The present invention uses a finger assignment system to prevent the assignment of receiver fingers to the same multipath component and to avoid the inefficiencies associated with current schemes. The finger assignment system accomplishes this by setting an offset or delay difference between the offsets or delays assigned to any two fingers, and if the offset or delay difference is violated, adjusting the offset(s) or delay(s) of one or both of the fingers to differ from each other by at least the offset difference. In certain embodiments, the receiver establishes an offset or delay difference between the offsets or delays assigned to any two fingers which are receiving signals from the same antenna. If the offset difference is violated, the finger assignment system adjusts the assigned offset or delay of the weaker finger to differ from the assigned offset of the stronger finger by at least the offset difference. Thus, the receiver scheme according to the principles of the present invention tends to have more receiver fingers tracking more multipath components a higher percentage of the time. The more effective use of the receiver fingers according to the principles of the present invention tends to increase the multipath diversity of the receiver and to improve the average SNR of the receiver.